Fin field effect transistor including a strained epitaxial semiconductor shell

ABSTRACT

A semiconductor fin including a single crystalline semiconductor material is formed on a dielectric layer. A semiconductor shell including an epitaxial semiconductor material is formed on all physically exposed surfaces of the semiconductor fin by selective epitaxy, which deposits the semiconductor material only on semiconductor surfaces and not on dielectric surfaces. The epitaxial semiconductor material can be different from the single crystalline semiconductor material, and the semiconductor shell can be bilaterally strained due to lattice mismatch. A fin field effect transistor including a strained channel can be formed. Further, the semiconductor shell can advantageously alter properties of the source and drain regions, for example, by allowing incorporation of more dopants or by facilitating a metallization process.

BACKGROUND

The present disclosure generally relates to semiconductor devices, andparticularly to field effect transistors having a strained epitaxialsemiconductor shell, and methods of manufacturing the same.

Fin field effect transistors are employed in advanced semiconductorchips to provide higher on current per unit area as well as tightchannel control. Increase in the on-current per unit area can thus beachieved in conjunction with reduction in the leakage current in theoff-state with fin field effect transistors. A semiconductor fingenerally includes a same semiconductor material throughout. Thus, thematerial properties of the channel of each fin field effect transistorare determined by the material of the semiconductor fin of the fin fieldeffect transistor.

SUMMARY

A semiconductor fin including a single crystalline semiconductormaterial is formed on a dielectric layer. A semiconductor shellincluding an epitaxial semiconductor material is formed on allphysically exposed surfaces of the semiconductor fin by selectiveepitaxy, which deposits the semiconductor material only on semiconductorsurfaces and not on dielectric surfaces. The epitaxial semiconductormaterial can be different from the single crystalline semiconductormaterial, and the semiconductor shell can be bilaterally strained due tolattice mismatch. A fin field effect transistor including a strainedchannel can be formed. Further, the semiconductor shell canadvantageously alter properties of the source and drain regions, forexample, by allowing incorporation of more dopants or by facilitating ametallization process.

According to an aspect of the present disclosure, a method of forming asemiconductor structure is provided. A semiconductor fin including afirst semiconductor material is formed on an insulator layer. Thesemiconductor fin laterally is bounded by a pair of parallel lengthwisesidewalls and a pair of parallel widthwise sidewalls. An epitaxialsemiconductor shell including a second semiconductor material is formedwith epitaxially alignment with the first semiconductor material on allsurfaces of the pair of parallel lengthwise sidewalls and the pair ofparallel widthwise sidewalls.

According to another aspect of the present disclosure, a semiconductorstructure includes an insulator layer located in a substrate, and asemiconductor fin including a first semiconductor material. Thesemiconductor fin is laterally bounded by a pair of parallel lengthwisesidewalls and a pair of parallel widthwise sidewalls, and is located onthe insulator layer. The semiconductor structure further includes anepitaxial semiconductor shell that contains a second semiconductormaterial that is epitaxially aligned to the first semiconductormaterial. The epitaxial semiconductor shell contiguously contacts all ofthe pair of parallel lengthwise sidewalls and all of the pair ofparallel widthwise sidewalls.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a top-down view of an exemplary semiconductor structure afterrecessing a portion of a top semiconductor layer according to anembodiment of the present disclosure.

FIG. 1B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 1A along the vertical plane B-B′ of FIG.1A.

FIG. 2A is a top-down view of the exemplary semiconductor structureafter formation of an epitaxial semiconductor material layer having adifferent composition than the top semiconductor layer according to anembodiment of the present disclosure.

FIG. 2B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 2A along the vertical plane B-B′ of FIG.2A.

FIG. 3A is a top-down view of the exemplary semiconductor structureafter planarizing top surfaces of the top semiconductor layer and theepitaxial semiconductor material layer according to an embodiment of thepresent disclosure.

FIG. 3B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 3A along the vertical plane B-B′ of FIG.3A.

FIG. 4A is a top-down view of the exemplary semiconductor structureafter forming fin mask structures according to an embodiment of thepresent disclosure.

FIG. 4B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 4A along the vertical plane B-B′ of FIG.4A.

FIG. 5A is a top-down view of the exemplary semiconductor structureafter formation of semiconductor fins according to an embodiment of thepresent disclosure.

FIG. 5B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 5A along the vertical plane B-B′ of FIG.5A.

FIG. 6A is a top-down view of the exemplary semiconductor structureafter formation of a first dielectric mask layer and a first patternedphotoresist layer according to an embodiment of the present disclosure.

FIG. 6B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 6A along the vertical plane B-B′ of FIG.6A.

FIG. 7A is a top-down view of the exemplary semiconductor structureafter patterning the first dielectric mask layer and performing a firstselective epitaxy process according to an embodiment of the presentdisclosure.

FIG. 7B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 7A along the vertical plane B-B′ of FIG.7A.

FIG. 8A is a top-down view of the exemplary semiconductor structureafter removal of the first dielectric mask layer and formation of asecond dielectric mask layer and a second patterned photoresist layeraccording to an embodiment of the present disclosure.

FIG. 8B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 8A along the vertical plane B-B′ of FIG.8A.

FIG. 9A is a top-down view of the exemplary semiconductor structurepatterning the second dielectric mask layer and performing a secondselective epitaxy process according to an embodiment of the presentdisclosure.

FIG. 9B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 9A along the vertical plane B-B′ of FIG.9A.

FIG. 10A is a top-down view of the exemplary semiconductor structureafter removal of the second dielectric mask layer according to anembodiment of the present disclosure.

FIG. 10B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 10A along the vertical plane B-B′ ofFIG. 10A.

FIG. 11A is a top-down view of the exemplary semiconductor structureafter formation of source and drain regions and gate electrodesaccording to an embodiment of the present disclosure.

FIG. 11B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 11A along the vertical plane B-B′ ofFIG. 11A.

FIG. 12A is a top-down view of the exemplary semiconductor structureafter formation of a contact level dielectric layer and various contactvia structures according to an embodiment of the present disclosure.

FIG. 12B is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 12A along the vertical plane B-B′ ofFIG. 12A.

FIG. 13A is a top-down view of a variation of the exemplarysemiconductor structure after formation of raised source and drainregions according to an embodiment of the present disclosure.

FIG. 13B is a vertical cross-sectional view of the variation of theexemplary semiconductor structure of FIG. 13A along the vertical planeB-B′ of FIG. 13A.

FIG. 14A is a top-down view of the variation of the exemplarysemiconductor structure after formation of a contact level dielectriclayer and various contact via structures according to an embodiment ofthe present disclosure.

FIG. 14B is a vertical cross-sectional view of the variation of theexemplary semiconductor structure of FIG. 14A along the vertical planeB-B′ of FIG. 14A.

DETAILED DESCRIPTION

As stated above, the present disclosure relates to field effecttransistors having a strained epitaxial semiconductor shell, and methodsof manufacturing the same. Aspects of the present disclosure are nowdescribed in detail with accompanying figures. Like and correspondingelements are referred to by like reference numerals. Proportions ofvarious elements in the accompanying figures are not drawn to scale. Asused herein, ordinals such as “first” and “second” are employed merelyto distinguish similar elements, and different ordinals may be employedto designate a same element in the specification and/or claims.

Referring to FIGS. 1A and 1B, an exemplary semiconductor structure ofthe present disclosure includes a semiconductor substrate that containsa semiconductor material layer 30L. The semiconductor material layer 30Lcan be a top semiconductor layer located within asemiconductor-on-insulator (SOI) substrate, or can be an upper portionof a bulk semiconductor substrate. The semiconductor material layer 30Lcan be located on a substrate 8, which can be a stack of a buriedinsulator layer 20 and a handle substrate 8, or can be a lower portionof a bulk substrate. The semiconductor material layer 30L includes asemiconductor material, which is herein referred to as a firstsemiconductor material. As provided, the semiconductor material layer30L may have a uniform thickness throughout. The thickness of thesemiconductor material layer 30L can be in a range from 5 nm to 500 nm,although lesser and greater thicknesses can also be employed.

In one embodiment, the first semiconductor material can be a singlecrystalline semiconductor material. The first semiconductor material canbe an elemental semiconductor material, an alloy of at least twoelemental semiconductor materials, a III-V compound semiconductormaterial, a II-VI compound semiconductor material, or an organicsemiconductor material. In one embodiment, the first semiconductormaterial can be selected from single crystalline silicon, a singlecrystalline silicon-germanium alloy, a single crystalline silicon-carbonalloy, and a single crystalline silicon-germanium-carbon alloy. In oneembodiment, the first semiconductor material can be single crystallinesilicon.

A dielectric mask layer 32 is formed over the top surface of thesemiconductor material layer 30L. The dielectric mask layer 32 includesa dielectric material such as silicon oxide, silicon nitride, siliconoxynitride, a dielectric metal oxide, or a combination thereof. Aphotoresist layer 37 is applied over the dielectric mask layer 32, andis lithographically patterned to cover the dielectric material layer 32within a first device region 100A, while the photoresist layer 37 isremoved from a second device region 100B.

The dielectric mask layer 32 is etched employing the photoresist layer37 as an etch mask. An anisotropic etch or an isotropic etch may beemployed to etch physically exposed portions of the dielectric masklayer 32. Subsequently, an upper portion of the semiconductor materiallayer 30L within the area that is not covered by the photoresist layer37 is etched employing the photoresist layer 37 and/or the dielectricmask layer 32 as an etch mask. A recessed region 39 is formed within thesecond device region 100B.

If the semiconductor material layer 30L is provided over a buriedinsulator layer 20, the depth of recess in the recessed region 39 isselected to be less than the thickness of the semiconductor materiallayer 30L is less than the thickness of the semiconductor material layer30L. In general, the recess depth is controlled such that the bottomsurface of the recessed region 39 is a planar surface of the firstsemiconductor material within the semiconductor material layer 30L,which can be a top semiconductor layer of an SOI substrate or an upperportion of a bulk semiconductor substrate. The photoresist layer 37 issubsequently removed, for example, by ashing.

Referring to FIGS. 2A and 2B, an epitaxial semiconductor material layer40L is deposited on the physically exposed bottom surface and sidewallsof the recessed region 39 (See FIG. 1B) by selective epitaxy. Theepitaxial semiconductor material layer 40L includes a single crystallinesemiconductor material having a different composition than firstsemiconductor material.

In one embodiment, the semiconductor material of the epitaxialsemiconductor material layer 40L is lattice mismatched with respect tothe first semiconductor material. In one embodiment, the firstsemiconductor material and the semiconductor material of the epitaxialsemiconductor material layer 40L can be two semiconductor materialshaving different semiconductor compositions and selected from singlecrystalline silicon, single crystalline silicon-germanium alloys, singlecrystalline silicon-carbon alloys, and a single crystallinesilicon-germanium-carbon alloy. As used herein, the “semiconductorcomposition” of semiconductor material refers to the component of thecomposition of the semiconductor material that excludes electricaldopants, which can be p-type dopants such as B, Ga, or As and/or n-typedopants such as P, As, or Sb. In other words, the semiconductorcomposition of a semiconductor material is a composition of an intrinsicsemiconductor material obtained from the semiconductor material byremoval of all electrical dopants. In one embodiment, the firstsemiconductor material is single crystalline silicon, and thesemiconductor material of the epitaxial semiconductor material layer 40Lcan be a single crystalline silicon-germanium alloy. A top surface ofthe epitaxial semiconductor material layer 40L may contact a sidewall ofthe dielectric mask layer 32, or a portion of the epitaxialsemiconductor material layer 40L may overlie a peripheral the epitaxialsemiconductor material layer 40L.

Referring to FIGS. 3A and 3B, the dielectric mask layer 32 is removedselective to the semiconductor materials of the semiconductor materiallayer 30L and the epitaxial semiconductor material layer 40L. If thedielectric mask layer 32 includes silicon oxide, a wet etch employinghydrofluoric acid can be employed to remove the dielectric mask layer 32selective to the semiconductor materials of the semiconductor materiallayer 30L and the epitaxial semiconductor material layer 40L. If thedielectric mask layer 32 includes silicon nitride, a wet etch employinghot phosphoric acid can be employed to remove the dielectric mask layer32 selective to the semiconductor materials of the semiconductormaterial layer 30L and the epitaxial semiconductor material layer 40L.

Optionally, a planarization process can be employed to planarize the topsurfaces of the semiconductor material layer 30L and the epitaxialsemiconductor material layer 40L. In other words, the top surface of oneof the semiconductor material layer 30L and the epitaxial semiconductormaterial layer 40L that protrudes above the top surface of the other ofthe semiconductor material layer 30L and the epitaxial semiconductormaterial layer 40L can be recessed during the planarization process sothat the top surfaces of the semiconductor material layer 30L and theepitaxial semiconductor material layer 40L become coplanar with eachother.

Optionally, a thermal anneal can be performed to diffuse thesemiconductor materials of the semiconductor material layer 30L and theepitaxial semiconductor material layer 40L across the interface betweenthe semiconductor material layer 30L and the epitaxial semiconductormaterial layer 40L. In this case, the composition of the semiconductormaterials in the second device region 100B can be homogenized, and thevolume of the epitaxial semiconductor material layer 40L may expandvertically and laterally so that the epitaxial semiconductor materiallayer 40L contacts the top surface of the buried insulator layer 20. Theepitaxial semiconductor material layer 40L can have a portion having ahomogeneous composition within the second device region 100B.

Referring to FIGS. 4A and 4B, fin mask structures 70 are formed on thetop surfaces of the semiconductor material layer 30L and the epitaxialsemiconductor material layer 40L. In one embodiment, the fin maskstructures 70 can be patterned portions of a dielectric material layer.The fin mask structures 70 can be formed, for example, by deposition andpatterning of a dielectric material layer. The dielectric material layerincludes a dielectric material such as silicon oxide, silicon nitride, adielectric metal oxide, or a combination thereof. The dielectricmaterial layer can be deposited by chemical vapor deposition (CVD) oratomic layer deposition (ALD). The patterning of the dielectric materiallayer can be performed, for example, by application and lithographicpatterning of a photoresist layer, and transfer of the patterns in thephotoresist layer into the dielectric material layer by an etch, whichcan be an anisotropic etch such as a reactive ion etch. Remainingportions of the photoresist layer can be removed, for example, byashing. The thickness of the fin mask structures 70 can be in a rangefrom 1 nm to 50 nm, although lesser and greater thicknesses can also beemployed.

In another embodiment, the fin mask structures 70 can be a patternedphotoresist layer. In this case, a photoresist layer can be applied overthe semiconductor material layer 30L and the epitaxial semiconductormaterial layer 40L, and can be lithographically patterned to form thefin mask structures.

In one embodiment, at least one of the fin mask structures 70 can have arectangular horizontal cross-sectional shape, i.e., a horizontalcross-sectional shape of a rectangle. In one embodiment, a plurality offin mask structures 70 having rectangular horizontal cross-sectionalshapes can be formed.

Referring to FIGS. 5A and 5B, the patterns in the fin mask structures 70can be transferred into the semiconductor material layer 30L and theepitaxial semiconductor material layer 40L by an anisotropic etch thatemploys the fin mask structures 70 as an etch mask. Each patternedportion of the semiconductor material layer 30L constitutes asemiconductor fin, which is herein referred to as a first semiconductorfin 30. Each patterned portion of the epitaxial semiconductor materiallayer 40L constitutes a semiconductor fin, which is herein referred toas a second semiconductor fin 40.

As used herein, a “semiconductor fin” refers to a semiconductor materialportion having a pair of parallel lengthwise sidewalls that arelaterally spaced by a uniform dimension. In one embodiment, eachsemiconductor fin (30, 40) can have a rectangular horizontalcross-sectional shape such that the spacing between the pair of parallellengthwise sidewalls is the same as the length of shorter sides of theshape of the rectangular horizontal cross-sectional shape. As usedherein, a “fin field effect transistor” refers to a field effecttransistor in which at least a body region is located within asemiconductor fin.

In one embodiment, each of the semiconductor fins can have a rectangularhorizontal cross-sectional shape bounded by a pair of parallellengthwise sidewalls and a pair of parallel widthwise sidewalls. As usedherein, a “lengthwise direction” of an element refers to a directionabout which the moment of inertia of the element becomes a minimum. Asused herein, a lengthwise sidewall of an element refers to a sidewall ofan element that extends along the lengthwise direction of the element.As used herein, a widthwise sidewall of an element refers to a sidewallof the element that extends along a horizontal direction that isperpendicular to the lengthwise direction of the element.

In one embodiment, the fin mask structures 70 can have rectangularhorizontal cross-sectional shapes. Each first semiconductor fin 30 canbe laterally bounded by a pair of parallel lengthwise sidewalls and apair of parallel widthwise sidewalls. Each second semiconductor fin 40can be laterally bounded by a pair of parallel lengthwise sidewalls anda pair of parallel widthwise sidewalls. Each semiconductor fin 30 canhave a planar top surface having a periphery that coincides with topedges of the pair of parallel lengthwise sidewalls and the pair ofparallel widthwise sidewalls of the semiconductor fin 30. In oneembodiment, the top surfaces of the semiconductor fins (30, 40) can becoplanar, i.e., be located within a same horizontal plane.

If the semiconductor fins (30, 40) are formed by patterning an upperportion of a bulk semiconductor substrate, a shallow trench isolationlayer including a dielectric material can be formed around a lowerportion of each semiconductor fin (30, 40). The shallow trench isolationlayer can be formed by deposition of a dielectric material layer,planarization of the dielectric material layer, and recessing of thedielectric material layer relative to the top surfaces of thesemiconductor fins (30, 40). Alternately, the shallow trench isolationlayer can be formed by spin-on coating of a flowable dielectric materialsuch as spin-on oxide (SOG).

Referring to FIGS. 6A and 6B, a first dielectric mask layer 72 is formedon the top surfaces and the sidewalls of the semiconductor fins (30, 40)and the top surface of the buried insulator layer 20 (or a portion of abulk semiconductor substrate underlying the semiconductor fins). Thefirst dielectric mask layer 72 includes a dielectric material such assilicon oxide, silicon nitride, silicon oxynitride, dielectric metaloxide, or a combination thereof. The first dielectric mask layer 72 isformed by a conformal deposition method such as chemical vapordeposition (CVD) or atomic layer deposition (ALD). The thickness of thefirst dielectric mask layer 72 can be in a range from 1 nm to 30 nm,although lesser and greater thicknesses can also be employed.

A first patterned photoresist layer 75 covering the second semiconductorfins 40 in the second device region 100B can be formed, for example, byapplication of a blanket (unpatterned) photoresist layer over the entirearea of the exemplary semiconductor structure, and subsequently removingportions of the applied photoresist layer from the first device region100A, while the portion of the photoresist layer within the seconddevice region 100B remains over the second semiconductor fins 40. Thus,the first patterned photoresist layer 75 can be present in the seconddevice region 100B, and not present in the first device region 100A.

Referring to FIGS. 7A and 7B, the unmasked portion of the firstdielectric mask layer 72 in the first device region 100A is removed byan etch, which can be an isotropic etch such as a wet etch. The firstpatterned photoresist layer 75 is subsequently removed, for example, byashing.

A selective epitaxy process is performed to deposit a single crystallinesemiconductor material on the physically exposed surfaces of the firstsemiconductor fins 30. The single crystalline semiconductor materialdeposited by the selective epitaxy process is herein referred to as asecond semiconductor material. The second semiconductor material is asemiconductor material that is different from the first semiconductormaterial in composition, and can be single crystalline silicon, a singlecrystalline silicon germanium alloy, a singe crystalline silicon carbonalloy, a single crystalline silicon germanium carbon alloy, or a singlecrystalline compound semiconductor material.

In one embodiment, the first semiconductor material can be singlecrystalline silicon, and the second semiconductor material can be asingle crystalline silicon germanium alloy, a single crystalline siliconcarbon alloy, or a single crystalline silicon germanium carbon alloy. Inone embodiment, the second semiconductor material has a different bandgap width than the first semiconductor material. In one embodiment, thefirst semiconductor material can be single crystalline silicon, and thesecond semiconductor material can be a single crystalline silicongermanium alloy.

Because the second semiconductor material is different from the firstsemiconductor material, the selective epitaxy process is a selectiveheteroepitaxy process, i.e., a selective epitaxy process that deposits amaterial different from the underlying material to a surface of theunderlying material. During the selective epitaxy process, at least onesemiconductor precursor gas and an etchant gas are flowed simultaneouslyor alternately into a processing chamber including the exemplarysemiconductor structure. The second semiconductor material is depositedonly on single crystalline surfaces such as the physically exposedsurfaces of the first semiconductor fins 30, and is not deposited onamorphous surfaces such as the surfaces of the first dielectric masklayer 72 and the buried insulator layer 20.

Each contiguous portion of the second semiconductor material depositeddirectly on the surfaces of a first semiconductor fin 30 constitutes acontiguous semiconductor material layer that contacts all surfaces of apair of parallel lengthwise sidewalls, a pair of parallel widthwisesidewalls, and a planar top surface of the first semiconductor fin 30,and is herein referred to as a first epitaxial semiconductor shell 50.As used herein, a “semiconductor shell” refers to a semiconductormaterial portion that laterally encloses another structure within acontiguous periphery defined by inner surfaces of the semiconductormaterial portion. Each first epitaxial semiconductor shell 50 is inepitaxially alignment with the first semiconductor material on allsurfaces of the pair of parallel lengthwise sidewalls, the pair ofparallel widthwise sidewalls, and the planar top surface of theunderlying first semiconductor fin 30. Each first epitaxialsemiconductor shell 50 is formed by selective epitaxy of the secondsemiconductor material. The thickness of each first epitaxialsemiconductor shell 50 can be in a range from 1% to 30% of the thicknessof the first semiconductor fin 30 that the first epitaxial semiconductorshell 50 contacts, and can be, for example, in a range from 1 nm to 30nm, although lesser and greater thicknesses can also be employed.

Because the second semiconductor material is lattice-mismatched withrespect to the first semiconductor material, each portion of a firstepitaxial semiconductor shell 50 has a biaxial strain within a planethat is parallel to a proximal interface with the first semiconductorfin 30. As used herein, a “proximal interface” refers to the interfacethat is the most proximate from the point of measurement or reference.

The exemplary semiconductor structure includes an insulator layerlocated in a substrate (10, 20). The insulator layer can be a buriedinsulator layer 20 derived from an SOI substrate, or can be a shallowtrench isolation layer including a dielectric material and formed over aremaining portion of a bulk semiconductor substrate after formation ofsemiconductor fins (30, 40). Each first semiconductor fin 30 can includethe first semiconductor material, and can be laterally bounded by a pairof parallel lengthwise sidewalls and a pair of parallel widthwisesidewalls, and located on the insulator layer. Each first epitaxialsemiconductor shell 50 includes the second semiconductor material thatis epitaxially aligned to the first semiconductor material, andcontiguously contacts all of the pair of parallel lengthwise sidewallsand all of the pair of parallel widthwise sidewalls.

In one embodiment, each first epitaxial semiconductor shell 50 can havethe same thickness on the pair of parallel lengthwise sidewalls and onthe pair of parallel widthwise sidewalls. Each first semiconductor fin30 can include a planar top surface having a periphery that coincideswith top edges of the pair of parallel lengthwise sidewalls and the pairof parallel widthwise sidewalls. Each first epitaxial semiconductorshell 50 can contact the entirety of the planar top surface of anunderlying first semiconductor fin 30.

The first dielectric mask layer 72 is subsequently removed selective tothe first epitaxial semiconductor shells 50 and the second semiconductorfins 40, for example, by a wet etch. For example, if the firstdielectric mask layer 72 includes silicon oxide, a wet etch employinghydrofluoric acid can be employed.

Referring to FIGS. 8A and 8B, a second dielectric mask layer 74 isformed on the top surfaces and the sidewalls of the semiconductor fins(30, 40) and the top surface of the buried insulator layer 20 (or aportion of a bulk semiconductor substrate underlying the semiconductorfins). The second dielectric mask layer 74 includes a dielectricmaterial such as silicon oxide, silicon nitride, silicon oxynitride,dielectric metal oxide, or a combination thereof. The second dielectricmask layer 74 is formed by a conformal deposition method such aschemical vapor deposition (CVD) or atomic layer deposition (ALD). Thethickness of the second dielectric mask layer 74 can be in a range from1 nm to 30 nm, although lesser and greater thicknesses can also beemployed.

A second patterned photoresist layer 77 covering the first semiconductorfins 30 and the first epitaxial semiconductor shells 50 in the firstdevice region 100A can be formed, for example, by application of ablanket (unpatterned) photoresist layer over the entire area of theexemplary semiconductor structure, and subsequently removing portions ofthe applied photoresist layer from the second device region 100B, whilethe portion of the photoresist layer within the first device region 100Aremains over the first semiconductor fins 30 and the first epitaxialsemiconductor shells 50. Thus, the second patterned photoresist layer 77can be present in the first device region 100A, and not present in thesecond device region 100B.

Referring to FIGS. 9A and 9B, the unmasked portion of the seconddielectric mask layer 74 in the second device region 100B is removed byan etch, which can be an isotropic etch such as a wet etch. The secondpatterned photoresist layer 77 is subsequently removed, for example, byashing.

Another selective epitaxy process is performed to deposit a singlecrystalline semiconductor material on the physically exposed surfaces ofthe second semiconductor fins 40. The single crystalline semiconductormaterial of the second semiconductor fins 40 is herein referred to as athird semiconductor material. The single crystalline semiconductormaterial deposited by the selective epitaxy process is herein referredto as a fourth semiconductor material. The fourth semiconductor materialis a semiconductor material that is different from the thirdsemiconductor material in composition, and can be single crystallinesilicon, a single crystalline silicon germanium alloy, a singlecrystalline silicon carbon alloy, a single crystalline silicon germaniumcarbon alloy, or a single crystalline compound semiconductor material.

In one embodiment, the third semiconductor material can be a singlecrystalline silicon germanium alloy, and the fourth semiconductormaterial can be single crystalline silicon, a single crystalline silicongermanium alloy having a different atomic concentration of germaniumthan the third semiconductor material, a single crystalline siliconcarbon alloy, or a single crystalline silicon germanium carbon alloy. Inone embodiment, the fourth semiconductor material has a different bandgap width than the second semiconductor material. In one embodiment, thefirst semiconductor material can be single crystalline silicon, thesecond semiconductor material can be a single crystalline silicongermanium alloy, the third semiconductor material can be another singlecrystalline silicon germanium alloy, and the fourth semiconductormaterial can be single crystalline silicon.

Because the fourth semiconductor material is different from the thirdsemiconductor material, the selective epitaxy process is a selectiveheteroepitaxy process. During the selective epitaxy process, at leastone semiconductor precursor gas and an etchant gas are flowedsimultaneously or alternately into a processing chamber including theexemplary semiconductor structure. The fourth semiconductor material isdeposited only on single crystalline surfaces such as the physicallyexposed surfaces of the second semiconductor fins 40, and is notdeposited on amorphous surfaces such as the surfaces of the seconddielectric mask layer 74 and the buried insulator layer 20.

Each contiguous portion of the fourth semiconductor material depositeddirectly on the surfaces of a second semiconductor fin 40 constitutes acontiguous semiconductor material layer that contacts all surfaces of apair of parallel lengthwise sidewalls, a pair of parallel widthwisesidewalls, and a planar top surface of the second semiconductor fin 40,and is herein referred to as a second epitaxial semiconductor shell 60.Each second epitaxial semiconductor shell 60 is in epitaxially alignmentwith the second semiconductor material on all surfaces of the pair ofparallel lengthwise sidewalls, the pair of parallel widthwise sidewalls,and the planar top surface of the underlying second semiconductor fin40. Each second epitaxial semiconductor shell 60 is formed by selectiveepitaxy of the fourth semiconductor material. The thickness of eachsecond epitaxial semiconductor shell 60 can be in a range from 1% to 30%of the thickness of the second semiconductor fin 40 that the secondepitaxial semiconductor shell 60 contacts, and can be, for example, in arange from 1 nm to 30 nm, although lesser and greater thicknesses canalso be employed.

Because the fourth semiconductor material is lattice-mismatched withrespect to the third semiconductor material, each portion of a secondepitaxial semiconductor shell 60 has a biaxial strain within a planethat is parallel to a proximal interface with the first semiconductorfin 30. In one embodiment, the first epitaxial semiconductor shells 50can have a compressive biaxial strain within a plane that is parallel toa proximal interface with a most proximate first semiconductor fin 30,and the second epitaxial semiconductor shells 60 can have a tensilebiaxial strain within a plane that is parallel to a proximal interfacewith a most proximate second semiconductor fin 40. In anotherembodiment, the first epitaxial semiconductor shells 50 can have atensile biaxial strain within a plane that is parallel to a proximalinterface with a most proximate first semiconductor fin 30 and thesecond epitaxial semiconductor shells 60 can have a compressive biaxialstrain within a plane that is parallel to a proximal interface with amost proximate second semiconductor fin 40. In one embodiment, eachepitaxial semiconductor shell (50 or 60) can be biaxially strained indirections parallel to a proximal interface with a most proximatesemiconductor fin (30 or 40).

Each second semiconductor fin 40 can include the third semiconductormaterial, and can be laterally bounded by a pair of parallel lengthwisesidewalls and a pair of parallel widthwise sidewalls, and located on theinsulator layer. Each second epitaxial semiconductor shell 60 includesthe fourth semiconductor material that is epitaxially aligned to thethird semiconductor material, and contiguously contacts all of the pairof parallel lengthwise sidewalls and all of the pair of parallelwidthwise sidewalls.

In one embodiment, each second epitaxial semiconductor shell 60 can havethe same thickness on the pair of parallel lengthwise sidewalls and onthe pair of parallel widthwise sidewalls. Each second semiconductor fin40 can include a planar top surface having a periphery that coincideswith top edges of the pair of parallel lengthwise sidewalls and the pairof parallel widthwise sidewalls. Each second epitaxial semiconductorshell 60 can contact the entirety of the planar top surface of anunderlying second semiconductor fin 40.

Referring to FIGS. 10A and 10B, the second dielectric mask layer 74 issubsequently removed selective to the first epitaxial semiconductorshells 50 and the second epitaxial semiconductor shells 60, for example,by a wet etch. For example, if the second dielectric mask layer 74includes silicon oxide, a wet etch employing hydrofluoric acid can beemployed.

Referring to FIGS. 11A and 11B, a gate dielectric layer and a gateconductor layer are deposited and lithographically patterned to formgate structures. The gate structures include a first gate structure(80A, 82A) formed in the first device region 100A and a second gatestructure (80N, 82B) formed in the second device region 100B. The firstgate structure (80A, 82A) includes a first gate dielectric 80A, which isa remaining portion of the gate dielectric layer, and a first gateelectrode 82A, which is a remaining portion of the gate conductor layer.The first gate structure (80A, 82A) straddles at least one integratedassembly of a first semiconductor fin 30 and a first epitaxialsemiconductor shell 50 (See FIGS. 10A and 10B). The second gatestructure (80B, 82B) includes a second gate dielectric 80B, which is aremaining portion of the gate dielectric layer, and a second gateelectrode 82B, which is a remaining portion of the gate conductor layer.The second gate structure (80B, 82B) straddles at least one integratedassembly of a second semiconductor fin 40 and a second epitaxialsemiconductor shell 60 (See FIGS. 10A and 10B). As used herein, an“integrated assembly” refers to a structure including at least twophysically contacting structures.

Dielectric gate spacers (84A, 84B) can be formed on sidewalls of each ofthe gate structures (80A, 82A, 80B, 82B), for example, by deposition ofa conformal dielectric material layer and an anisotropic etch. Thedielectric gate spacers (84A, 84B) can include, for example, a firstgate spacer 84A formed in the first device region 100A, and a secondgate spacer 84B formed in the second device region 100B.

Electrical dopants of a conductivity type (which can be p-type orn-type) can be implanted into the device regions (100A, 100B) to formvarious source and drain regions before, and/or after, formation of thedielectric gate spacers (84A, 84B). A first source region (30S, 505) anda first drain region (30D, 50D) can be formed in portions of the firstsemiconductor fins 30 and the first epitaxial semiconductor shell 50that are not masked by the first gate structure(s) (80A, 82A). A secondsource region (40S, 60S) and a second drain region (60D, 60D) can beformed in portions of the second semiconductor fins 40 and the secondepitaxial semiconductor shell 60 that are not masked by the second gatestructure(s) (80B, 82B). Unimplanted portions of each integral assemblyof a first semiconductor fin 30 and a first epitaxial semiconductorshell 50 constitutes a first body region (30B, 50B). Unimplantedportions of each integral assembly of a second semiconductor fin 40 anda second epitaxial semiconductor shell 60 constitutes a second bodyregion (40B, 60B).

Each first source region (30S, 50S) includes a first fin source portion30S that is a doped portion of a first semiconductor fin 30, and a firstshell source portion 50S that is a doped portion of a first epitaxialsemiconductor shell 50. Each first drain region (30D, 50D) includes afirst fin drain portion 30D that is a doped portion of a firstsemiconductor fin 30, and a first shell drain portion 50D that is adoped portion of a first epitaxial semiconductor shell 50. Each firstbody region (30B, 50B) includes a first fin body portion 30B that is anunimplanted portion of a first semiconductor fin 30, and a first shellbody portion 50B that is an unimplanted portion of a first epitaxialsemiconductor shell 50.

Each second source region (40S, 60S) includes a second fin sourceportion 40S that is a doped portion of a second semiconductor fin 40,and a second shell source portion 60S that is a doped portion of asecond epitaxial semiconductor shell 60. Each second drain region (40D,60D) includes a second fin drain portion 40D that is a doped portion ofa second semiconductor fin 40, and a second shell drain portion 60D thatis a doped portion of a second epitaxial semiconductor shell 60. Eachsecond body region (40B, 60B) includes a second fin body portion 40Bthat is an unimplanted portion of a second semiconductor fin 40, and asecond shell body portion 60B that is an unimplanted portion of a secondepitaxial semiconductor shell 60.

Referring to FIGS. 12A and 12B, a contact level dielectric layer 90 isdeposited over the gate structures (80A, 82A, 80B, 82B) and the integralassemblies of the semiconductor fins and the epitaxial semiconductorshells. The contact level dielectric layer 90 includes a dielectricmaterial such as silicon oxide, silicon nitride, silicon oxynitride,and/or porous or non-porous organosilicate glass. Optionally, the topsurface of the contact level dielectric layer 90 can be planarized.

Various contact via structures can be formed through the contact leveldielectric layer 90. The various contact via structures can include, forexample, a first source contact via structure 92S that provideselectrical contact to the first source regions (30S, 50S), a first draincontact via structure 92D that provides electrical contact to the firstdrain regions (30D, 50D), a first gate contact via structure 92G thatprovides electrical contact to the first gate electrode 82A, a secondsource contact via structure 94S that provides electrical contact to thesecond source regions (30S, 50S), a second drain contact via structure94D that provides electrical contact to the second drain regions (30D,50D), and a second gate contact via structure 94G that provideselectrical contact to the second gate electrode 82B.

Optionally, metal semiconductor alloy regions (82S, 82D, 84S, 84D) canbe formed on the physically exposed top surface of the various shellsource portions (50S, 60S) and the various shell drain portions (50D,60D), for example, by deposition of a metal layer and an anneal thatforms a metal semiconductor alloy (such as a metal silicide). Unreactedremaining portions of the metal semiconductor alloy can be removed, forexample, by a wet etch.

A first fin field effect transistor can be formed in the first deviceregion 100A, and a second fin field effect transistor can be formed inthe second device region 100B. In one embodiment, the firstsemiconductor material can be single crystalline silicon, and the secondsemiconductor material can be a single crystalline silicon germaniumalloy. In this case, the first fin field effect transistor can provideadvantages of devices having a silicon-germanium channel. For example,the silicon-germanium channel provided by the first shell bodyportion(s) 50B can provide a greater on-current than a channel having asame geometry and including silicon. In addition, because the siliconmaterial in the underlying first fin body portion(s) 30B has a greaterband gap than silicon germanium alloys, the off-current of the first finfield effect transistor can be reduced relative to devices having a bodyregion consisting of a silicon germanium alloy.

In one embodiment, the third semiconductor material can be a singlecrystalline silicon germanium alloy or a single crystalline siliconcarbon alloy, and the fourth semiconductor material can be singlecrystalline silicon. In this case, the second fin field effecttransistor can provide advantages of devices having a silicon surface inthe second source region(s) (40S, 60S) and the second drain region(s).For example, silicon without germanium or silicon can provide a lowercontact resistance upon metallization than a silicon germanium alloy ora silicon carbon alloy.

Referring to FIG. 3, a planarization dielectric layer 50 is depositedover the gate structures (70A, 72A, 70B, 72B, 70C, 72C), the variousgate spacers (80A, 80B, 80C), the various source regions (92A, 92B,92C), and the various drain regions (93A, 93B, 93C). The planarizationdielectric layer 50 includes a dielectric material, which can be aself-planarizing dielectric material such as a spin-on glass (SOG), or anon-planarizing dielectric material such as silicon oxide, siliconnitride, organosilicate glass, or combinations thereof. Theplanarization dielectric layer 50 is subsequently planarized, forexample, by chemical mechanical planarization (CMP) such that topsurfaces of the gate structures (70A, 72A, 70B, 72B, 70C, 72C) becomephysically exposed. In one embodiment, the planarized top surface of theplanarization dielectric layer 50 can be coplanar with the top surfacesof the gate structures (70A, 72A, 70B, 72B, 70C, 72C). The planarizationdielectric layer 50 laterally surrounds the first gate structure (70A,72A), the second gate structure (70B, 72B), and the third gate structure(70C, 72C).

Referring to FIGS. 13A and 13B, a variation of the exemplarysemiconductor structure can be derived from the exemplary semiconductorstructure of FIGS. 11A and 11B by forming raised source regions andraised drain regions by selective epitaxy of additional semiconductormaterials. For example, a first raised source region 52S and a firstraised drain region 52D can be formed on the physically exposed surfacesof the first fin source portions 505 and the first fin drain portions50D by selective epitaxy of a doped semiconductor material, or byselective epitaxy of an undoped semiconductor material and subsequentdoping of the deposited semiconductor material by ion implantation orplasma doping. Likewise, a second raised source region 62S and a secondraised drain region 62D can be formed on the physically exposed surfacesof the second fin source portions 60S and the second fin drain portions60D by selective epitaxy of a doped semiconductor material, or byselective epitaxy of an undoped semiconductor material and subsequentdoping of the deposited semiconductor material by ion implantation orplasma doping. Optionally, dielectric mask layers (not shown) may beemployed to mask various regions in which deposition of a semiconductormaterial is not desired during each of the selective epitaxy processes.

Referring to FIGS. 14A and 14B, the processing steps of FIGS. 12A and12B can be performed to form various contact via structures.

An epitaxial semiconductor shell of the present disclosure provides asurface semiconductor layer having different electrical and/ormetallurgical properties than the underlying semiconductor fin.Advantageous surface properties of the epitaxial semiconductor shell canbe employed in conjunction with advantageous bulk properties of theunderlying semiconductor fin to modulate on-current, off-current, and/orcontact resistance of metal semiconductor alloy regions formed on thesource regions and/or the drain regions of a field effect transistor.

While the present disclosure has been described in terms of specificembodiments, it is evident in view of the foregoing description thatnumerous alternatives, modifications and variations will be apparent tothose skilled in the art. Each of the various embodiments of the presentdisclosure can be implemented alone, or in combination with any otherembodiments of the present disclosure unless expressly disclosedotherwise or otherwise impossible as would be known to one of ordinaryskill in the art. Accordingly, the present disclosure is intended toencompass all such alternatives, modifications and variations which fallwithin the scope and spirit of the present disclosure and the followingclaims.

What is claimed is:
 1. A method of forming a semiconductor structurecomprising: forming a semiconductor fin including a first semiconductormaterial on an insulator layer, said semiconductor fin laterally boundedby a pair of parallel lengthwise sidewalls and a pair of parallelwidthwise sidewalls; and forming an epitaxial semiconductor shellcomprising a second semiconductor material with epitaxially alignmentwith said first semiconductor material on all surfaces of said pair ofparallel lengthwise sidewalls and said pair of parallel widthwisesidewalls, wherein said semiconductor fin further comprises a planar topsurface having a periphery that coincides with top edges of said pair ofparallel lengthwise sidewalls and said pair of parallel widthwisesidewalls, and said epitaxial semiconductor shell is formed directly onsaid planar top surface.
 2. The method of claim 1, wherein saidepitaxial semiconductor shell is formed by selective epitaxy of saidsecond semiconductor material.
 3. The method of claim 1, furthercomprising forming a stack of a gate dielectric and a gate electrodeover a combination of said semiconductor fin and said epitaxialsemiconductor shell.
 4. The method of claim 3, further comprisingimplanting electrical dopants into portions of said semiconductor finand said epitaxial semiconductor shell employing at least said stack asan implantation mask, wherein implanted portions of said semiconductorfin and said epitaxial semiconductor shell include a source region and adrain region.
 5. The method of claim 1, wherein said secondsemiconductor material is lattice mismatched with respect to said firstsemiconductor material, and said epitaxial semiconductor shell isbiaxially strained in directions parallel to a proximal interface withsaid semiconductor fin.
 6. The method of claim 1, wherein said first andsecond semiconductor materials are selected from single crystallinesilicon, a single crystalline silicon-germanium alloy, a singlecrystalline silicon-carbon alloy, and a single crystallinesilicon-germanium-carbon alloy.
 7. The method of claim 1, furthercomprising: forming another semiconductor fin including a thirdsemiconductor material on said insulator layer, said anothersemiconductor fin laterally bounded by another pair of parallellengthwise sidewalls and another pair of parallel widthwise sidewalls;and forming another epitaxial semiconductor shell comprising a fourthsemiconductor material with epitaxial alignment with said thirdsemiconductor material on all surfaces of said another pair of parallellengthwise sidewalls and said another pair of parallel widthwisesidewalls.
 8. The method of claim 7, wherein one of said epitaxialsemiconductor shell and said another epitaxial semiconductor shell has abiaxial compressive strain within a plane that is parallel to a proximalinterface with said semiconductor fin, and another of said epitaxialsemiconductor shell and said another epitaxial semiconductor shell has abiaxial tensile strain within a plane that is parallel to a proximalinterface with said another semiconductor fm.
 9. The method of claim 7,wherein said first semiconductor material is single crystalline silicon,said second semiconductor material is a single crystallinesilicon-germanium alloy, said third semiconductor material is anothersingle crystalline silicon-germanium alloy, and said fourthsemiconductor material is single crystalline silicon.
 10. A method offorming a semiconductor structure comprising: forming a semiconductorfin including a first semiconductor material on an insulator layer, saidsemiconductor fin laterally bounded by a pair of parallel lengthwisesidewalls and a pair of parallel widthwise sidewalls; forming anepitaxial semiconductor shell comprising a second semiconductor materialwith epitaxially alignment with said first semiconductor material on allsurfaces of said pair of parallel lengthwise sidewalls and said pair ofparallel widthwise sidewalls; forming another semiconductor finincluding a third semiconductor material on said insulator layer, saidanother semiconductor fin laterally bounded by another pair of parallellengthwise sidewalls and another pair of parallel widthwise sidewalls;and forming another epitaxial semiconductor shell comprising a fourthsemiconductor material with epitaxial alignment with said thirdsemiconductor material on all surfaces of said another pair of parallellengthwise sidewalls and said another pair of parallel widthwisesidewalls.